There are signs that more Islamic State inspired militants have been sent to Belgium and Europe, Belgian authorities said on Tuesday, maintaining the country's threat status at the second-highest level.

Belgium's alert level was cut to three from the maximum of four just two days after the March 22 attacks which killed 32 people at the airport and on the metro in Brussels. It has remained at that level since.

"There are indications that Islamic State has sent fighters to Europe and Belgium, the threat level which is currently at three will not go down," a spokesman of Belgium's crisis center said.

By maintaining the level three status, authorities indicate that an attack is possible and likely.

Prosecutors detained two men seen on security footage alongside the metro and airport suicide bombers on April 8, meaning all known Brussels bombings suspects are dead or in custody.